DC/DC converters are useful for several purposes. For high voltage DC (HVDC), DC/DC converters can be used to transfer power unidirectionally or bidirectionally between two DC grids.
U.S. Pat. No. 3,942,089 presents a transmission substation for transforming a high D.C. voltage over an intermediate three phase A.C. network into a lower D.C. voltage. The substation includes an arrangement of inverters and rectifiers for effecting the transformation. The inverters and rectifiers are connected to a common bus bar via respective transformers. The transformers are used to adjust and match the voltage levels of the converter bridges as well as for preventing flow of DC current through bus bar. However, when compared to a typical HVDC transformer, additional isolation is required because the transformer is floating and is subjected to high DC voltage stress in steady state on both sides. This implies significant cost and/or additional protection systems.